When I was installing my new fuel pump under my car, I noticed this cable wasn't connected to anything. I have no idea what it is or where it goes. Any ideas? Thanks!

It goes to one of two heater valves that control heat to the cab. These valves are mounted to the heat exchangers right before the opening to the longitudinal.

Heat valve control wire.

I politely suggest to change your gas lines to Stainless, looks like legacy lines (on the right) which have been known to leak and cause burn-downs.

Suggestion only.......ask me why I am writing this

Thanks!

I eventually want to do that. I want to get a car lift installed and that would be one of my first things to do.